1. FIELD OF THE INVENTION
The present invention relates to electric discharge lamps which have fluorescent material deposited on the envelope containing the discharge path, that is, fluorescent lamps.
2. DESCRIPTION OF THE PRIOR ART
Considerable research effort has been expended since fluorescent lamps became available, to develop a compact fluorescent light source with a screw-in plug as replacement for standard incandescent lamps. Inventive ingenuity has produced lamp designs not now being marketed because of manufacturing difficulties to mass produce due to involved internal shapes and an inability to lay down uniform luminescent coatings.
A recent U.S. Pat. No. 3,899,712, wraps a helical tube around a cone. The tube is fabricated from two pieces: a depressed groove in the cone, and a mating piece joined to the groove to complete the helical tube. This approach is slightly simpler than that taken by U.S. Pat. No. 3,296,480 where the helical path is formed by three elements: an inside and an outside cone, and an internal barrier forming the helix. U.S. Pat. No. 2,501,375 is similarly constructed, but more convoluted, in that the tube is led into the interior of the form on which the helical tube is wound. The forming of the tube and sealing of the two halves of the tube along the longitudinal edges are expensive and a major obstacle in manufacturing, fraught with prohibitive shrinkage.
U.S. Pat. No. 3,611,009 creates an annular fluorescent between glass panels with a ballast above, both held in a metal fixture with a screw-in plug. The discharge energizes the phosphor at its maximum lumen output for a limited area near the arc. U.S. Pat. No. 2,406,146 explored various designs by which the arc discharge could be constrained to a zigzag path, but did not address itself to manufacturing ease or to providing a lamp with a screw-in plug.
Somewhat earlier, U.S. Pat. No. 3,059,137 described a circline toroidal fluorescent with ballast and starter within the toroid, the fixture having a screw-in plug.
Another approach, U.S. Pat. No. 3,521,120, to a screw-in lamp, avoids the arc discharge and energizes the gas with a radio frequency field. To get sufficient energy into the lamp, the frequency is quite high. As a result, radio frequency interference creates problems severely limiting its application. There is also the additional complexity of generating sufficient wattage at frequencies much higher than the 60 Hz power line frequency.
Fluorescent panels have been described with partitions to confine the arc discharge to a zigzag path, as in U.S. Pat. No. 3,848,150, which uses T-shaped baffles in a cylindrical tube. Straight-edge partitions are used in U.S. Pat. No. 3,508,103. An early version of applying partitions is described in U.S. Pat. No. 2,306,628. The common difficulty in fabricating these devices is insuring a leakproof seal between the edges of the partitions and the glass envelope, especially since these partitions are added as a separate part to be joined to one or both walls of the envelope in the manufacturing process, a difficult, expensive, and not uniformly successful technique.
Coffin, in 1896, describes in U.S. Pat. No. 560,476, a coil around an electrode of a carbon arc lamp which stabilizes and confines the arc to reduce flicker. Kim describes that the same phenomenon, wherein an external magnetic coil produces a parallel magnetic field, thereby reducing the flickering and shifting of the glow discharge in a neon glass lamp, the basis of U.S. Pat. No. 3,335,311, issued Aug. 8, 1967. An internal magnet was utilized to accomplish a similar effect in xenon arcs by Thomsen, U.S. Pat. No. 3,723,782, of Mar. 27, 1973. More recently, Drop et al applied a magnetic field to shape the arc discharge in a metal halide lamp, U.S. Pat. No. 4,001,626 of Jan. 4, 1977. Mention must be made of magnetic confinement of plasmas in fusion research in Tokomaks, MHD generator and other devices as described in U.S. Pat. No. 3,916,034, dated Oct. 28, 1975 of Tsuchimoto, and U.S. Pat No. 3,369,140, dated Feb. 13, 1968 of Furth. The references quoted apply magnetic fields to confine an arc or plasma, not disperse the arc as in this invention.